1. Field of the Invention
This invention relates to optical fiber collimating lenses for coupling an optical beam in or out of optical communication components and devices, and more specifically to fiber collimating lenses that use a specialty single-mode fiber to expand and focus the light beam and provide long working distances.
2. Description of the Related Art
Graded refractive index (GRIN) lenses have been widely used in optical components and devices as collimating, focusing and coupling lenses. A GRIN lens has a refractive index whose value varies with radial distance from the axis of the lens. The refractive index of a square-law GRIN lens can be given by:n2(r)=n02[1−g2r2]  (1)where n0 is the refractive index on the axis of the lens, r is the radial distance from the central axis, and g is the focusing parameter. In the GRIN lens, rays follow sinusoidal paths until reaching the back surface of the lens. A light ray that has traversed one pitch has traversed one cycle of the sinusoidal wave that characterizes that lens. Most conventional GRIN lenses are constructed with quarter pitch (¼ length of one cycle of the sinusoidal wave) length. The lens diameter (>1 mm) is a lot larger than the fiber diameter (125 microns), which complicates packaging and impacts reliability. Furthermore, the GRIN lenses are made by an ion exchange method, which is usually toxic and expensive.
Using a short length of a graded-index (GRIN) multimode optical fiber as a lens provides several advantages over the use of conventional GRIN lenses: small size, low cost, a common fiber/lens diameter, and a stable (fused) fiber/lens interface. The only limiting factors are the amount of beam expansion and the length of the working distance that can be achieved.
U.S. Pat. No. 4,701,011, issued to W. L. Emkey on Oct. 20, 1987, discloses a method of fusing an appropriate length GRIN fiber to the endface of a single-mode fiber to replace conventional quarter-pitch GRIN lens. The GRIN fiber is thus used as a lens, but provides many advantages over conventional quarter-pitch GRIN lenses epoxied to the fiber endfaces. In particular, the misalignment associated with the epoxied arrangement is reduced since the GRIN fiber can be chosen to comprise the same outer diameter as the single mode fiber. Additionally, the use of a section of optical fiber as a lens allows for a fused connection to be used instead of an epoxied connection, which results in a more stable and rugged interface between the fiber and the lens. The fabrication method and performance analysis for the lens were also reported by W. L. Emkey in Journal of Lightwave Technology, LT-5(9), 1987, pp. 1156–1164.
U.S. Pat. No. 6,542,665 B2, issued to W. A. Reed on Apr. 1, 2003, discloses a method to improve the parameters of the GRIN fiber lens. Specifically, the GRIN fiber lens has a radial profile for the refractive index that increases the Rayleigh range of the emitted beam. The increased Rayleigh range improves beam collimation so that the fiber may couple to other optical device over larger distance ranges.
U.S. Pat. No. 6,014,483, issued to M. Thual on Jan. 11, 2000, disclosed a method that includes fusing a fracture of pure silica coreless fiber of a specified length between the GRIN fiber and the single-mode fiber. In the Thual patent, the role of the silica is to spatially extend the output light beam from the single-mode fiber. The length must be carefully controlled to ensure the diameter of the beam is not larger than the fiber. This extension makes it possible to use the entire graded-index volume. The graded-index section is then exploited to the maximum extent as a lens since the volume occupied by the light beam is equal to the graded-index volume. With a silica section interposed between the graded-index lens and the single-mode fiber, the maximum mode diameter is 80 μm instead of 28 μm and the maximum distance between fibers is 1.8 mm instead of 460 μm, the coupling loss being equivalent in both cases (0.5 dB).
U.S. Pat. No. 5,774,607, issued to K. Shiraishi on Jun. 30, 1988, discloses a method of forming a fiber lens by fusing a square-law index fiber between the single-mode fiber and a coreless fiber having a hemispherical surface. The method was analyzed by K. Shiraishi in Journal of Lightwave Technology, 15(2), 1997, pp. 356–363.
U.S. Pat. No. 6,594,419 B2, issued to L. Ukrainczyk on Jul. 15, 2003, discloses a fiber lens formed by joining a single-mode fiber to a tapered graded-index multimode fiber as shown in FIG. 4 of the patent.
U.S. Pat. No. 6,130,972, issued to K. Shiraishi on Oct. 10 2000, discloses a fiber lens formed by cascaded joining of a single mode fiber with two graded index multimode fiber which have different square-low index distributions. The second multimode fiber may either have a convex distal end surface or be connected with an isotropic coreless fiber having a convex distal end. The method was also reported by K. Shiraishi, etc. in Journal of Lightwave Technology, 18(6), 2000, pp. 787–2000
U.S. Pat. No. 5,384,874 issued to S. Hirai on Jan. 24, 1995 discloses a fiber lens. In the Hirai patent a rod lens having a diameter not largely different from an outer diameter of an optical fiber is integrally connected to an optical single mode fiber. If the diameter of the graded-index optical fiber is larger than the single mode fiber, more of the light transmitted through the latter fiber can be entered into the former or a larger-diameter beam of mutually parallel rays can be obtained and it is easier to confirm a reference point for setting the length of the former fiber at the cutoff thereof.
There remains an acute need for a GRIN fiber lens that can expand the light beam to larger diameters and increase the working distance.